The invention relates to limited rotation motors such as galvanometers, and particularly relates to limited rotation torque motors used to drive optical elements such as mirrors for the purpose of guiding light beams in scanners.
Limited rotation torque motors generally include stepper motors and galvanometer motors. Certain stepper motors are well suited for applications requiring high speed and high duty cycle sawtooth scanning at large scan angles. For example, U.S. Pat. No. 6,275,319 discloses an optical scanning device for raster scanning applications that includes a multi-pole moving magnet rotor and a stator formed of a large number of rings (laminations), each of which includes radially directed teeth having individual coils wound around the teeth.
Limited rotation torque motors for certain applications, however, require the rotor to move between two positions with a precise and constant velocity rather than by stepping and settling in a sawtooth fashion. Such applications require that the time needed to reach the constant velocity be as short as possible and that the amount of error in the achieved velocity be as small as possible. To achieve this, a very high torque constant must be provided by the motor requiring as high a flux density as possible. This generally requires that the number of coil turns in the gap between the rotor and the stator be maximized without increasing the size of the gap.
Galvanometer motors generally provide a higher torque constant and typically include a rotor and drive circuitry for causing the rotor to rotate through a limited rotation range about a central axis in a controlled manner. A position transducer, e.g., a tachometer or a position sensor, and a feedback circuit coupled to the transducer permit the rotor to be driven by the drive circuitry responsive to an input signal and a feedback signal. For example, U.S. Pat. No. 5,225,770 discloses a conventional two-pole galvanometer motor, which is described below and shown herein in FIGS. 1–4 labeled prior art. The two-pole galvanometer includes a solid magnetic rotor 10 that is captured between two end portions 12 and 14 that in turn are coupled to two shafts 16 and 18 as shown in FIG. 1. The rotor 10 is polarized into essentially two semi-cylindrical magnetic portions 6 and 8 having opposite magnetic polarity, e.g., N and S. As shown in FIGS. 2–4, a stator for use with the rotor 10 of FIG. 1 may include two stator coil portions 22 and 24 and a magnetically permeable stator housing or backiron 13. The coil portions 22, 24 are attached to the housing 13 and disposed on opposite sides of the rotor within an annular space or gap 11 formed between the housing 13 and the rotor 10 such that the rotor is free to rotate about the axis 20 while the stator remains stationary.
A shown in FIG. 3 the coil portions 22 and 24 subtend a half angle of α0. In conventional galvanometers, the half angle α0 limits the angle of rotation of the rotor, which is generally limited to about 23 degrees. As the rotor rotates, however, the rotor poles N & S rotate toward the coil portion half angles α0 such that a portion of each semi-cylindrical magnetic section 6 and 8 is facing open space 27 between the opposing coils. In this case, the system is said to be underhung meaning that in extreme rotational positions of the rotation of the rotor 10, part of the rotor 10 is not opposed by coil windings 22 and 24. Since an underhung system has fewer coil windings available to drive the rotor 10 at the extreme edges of rotor rotation, there is less torque available to drive the rotor 10 at the edges of the travel. This results in lower acceleration of the rotor 10 at precisely the regions where high acceleration is desirable. If the number of stator coils is increased to fill the open space 27 such that the coils extend circumferentially further than the magnetic sections, then the system is overhung. This increases the torque available at the extreme edges of travel but at the expense of compromising other performance characteristics. In particular, the additional coil winding increase the overall coil resistance thereby increasing the heat that must be dissipated from the system. Accordingly, a prior art two coil galvanometer has an optimum number and configuration of stator coils that may be placed in the gap to drive the motor for providing sufficient torque while not exceeding heat dissipating capacity.
Another problem with conventional galvanometer systems, however, is fringing. As shown in FIG. 4, a plurality of flux lines 29 show the flux path of a conventional solid magnet two-pole galvanometer. Following the flux lines 29, a magnetic flux passes from the N pole 6 of the solid magnetic rotor 10, across the gap 11 between the magnetic rotor 10 and a backiron 13, circumferentially around the backiron 13, across the gap 11 and a second time to the S pole 8 of the solid magnet rotor 10 and then through the rotor returning to the N pole portion. As will be readily understood, the magnetic permeability of the magnet portions 6 and 8 and the backiron 13 may be many thousands of times greater than the magnetic permeability of the air and copper of the coils windings (shown in FIG. 3) that are present in the gap 11. Accordingly, there is a high reluctance or resistance to the flow of magnetic flux passing in the gap 11. As a result of the high reluctance in the gap 11, fringing occurs near the boundary 21 between the magnetic N and S portions 6 and 8. This condition is shown in FIGS. 3 & 4 wherein local flux lines 23 pass from one magnetic section to another without passing through any of the stator coil windings 22 and 24. Accordingly, the magnetic portions N and S, 6 and 8, near the border 27 do not contribute to generating torque for rotating the rotor 10. Since this flux never passes through a wire, it is lost to the torque-producing process. It is estimated that more than 15% of the magnetic volume near the equator is ineffective in producing torque for this reason, although the local magnet mass contributes excessively to the moment of inertia of the rotor since it is all positioned far from the axis of rotation. Accordingly, in spite of occupying 360 degrees of the rotor surface, only about 270 degrees of magnetic material is effective in producing torque such that a conventional two pole galvanometer configuration contains a large volume of space in the gap 11 between the edges of the two stator coils that produces no useful flux.
There are applications in which it is desirable to have greater torque than may be provided by conventional limited rotation torque motors. There is a need therefore, for limited rotation torque motors that provide improved flux density without adversely affecting the performance of the motor.